While Snow Leopard includes some improvements in the area of security, noted security researcher Charlie Miller, winner of two consecutive "Pwn2own" hacker contests and co-author of The Mac Hacker's Handbook, concludes that Apple missed the boat on security in Mac OS X Snow Leopard. "Snow Leopard's more secure than Leopard, but it's not as secure as Vista or Windows 7," Miller said.

This has nothing to do with the number of attacks on Windows or the lack thereof on OS X, it's Apple apparently failing to take security as seriously as they should. Since Leopard had a half-assed implementation I thought SL was bound to do it properly.

As was stated in a previous comment, Apple has taken security seriously. They've made strides to protect their binaries and libraries the stack and heap, etc. Even with all of the security mechanisms in place the system is able to be compromised. Adding further ASLR while beneficial of course, is not a magic bullet, nor does it mean that Apple does not take security seriously. They could have done it. I'm sure there are also valid technical reasons why they chose not to. Keep in mind that with much tighter security mechanisms available in Windows for years Windows systems have still been subject to compromise. Besides, the user is a far weaker link in the security chain and also a much larger and more likely target than the OS.